Nematodes are among the more economically-damaging plant parasites. At least 150 of as many as 10,000 species of nematodes are known to adversely effect plant life. Nematode feeding causes hypertrophy or gall formation on the infested plant. Evidence of heavy infestation includes plant stunting, pale foliage, wilting, and even plant death in extreme cases. Virtually all crops and ornamental plants can be attacked by parasitic nematodes.
Particularly destructive nematode species include the root knot nematodes which are hosted by tomatoes, alfalfa, cotton, corn, potatoes, citrus and many other crops; the golden nematode of potatoes; the sugar beet cyst nematode; and the citrus nematode. These and other species are described in "The Soil Pest Complex," Agricultural and Food Chemistry, Vol. 3, pages 202-205 (1955). This article also describes another complication resulting from nematode infestation, namely, lowered resistance to the effects of plant attack by bacteria and pathogenic soil fungi.
Only soil sterilization, which is practical only for small amounts of soil, is effective at completely eliminating nematodes. Previous attempts to limit parasite populations to agriculturally acceptable levels include soil fumigation, crop rotation using non-hosting plant varieties, and (to a much lesser extent) the development of plants which are resistant to infestation. In many instances, adequate control of nematode populations is achieved only by combinations of these techniques. Most parasite control programs directed to nematodes have proven quite costly.
Another serious problem in agriculture is the attack on plants by pathogenic microorganisms, particularly fungi. Such pathogens typically have been controlled by fumigating target plants with biocides prior to crop planting. Many of these biocides are no longer regarded as environmentally safe. Currently available fungicides are very expensive and lose effectiveness against successive generations of fungi due to rapid genetic adaptability of the fungi.
Carbon disulfide has been used as a soil fumigant as early as the 1870's to control the sugar beet nematode. Carbon disulfide also has proven effective as an insecticide, as a rodenticide, and for controlling certain weeds. Carbon disulfide is commercially impractical, however, because very large quantities must be applied due to its high volatility. Other drawbacks include its high flammability, objectionable odor, and toxicity to humans. When sold for fumigant use, carbon disulfide normally is mixed with fire retardants, such as carbon tetrachloride. Typically, such fumigant compositions contain no more than about 20 wt % carbon disulfide.
Numerous compositions have been developed which exhibit nematocidal properties. Active ingredients in such compositions include the polyamines described in U.S. Pat. No. 2,979,434 to Santmyer, the heterocyclic compounds described in U.S. Pat. No. 2,086,907 to Hessel, and various halogenated compounds such as 1,2-dibromoethane, methyl bromide, 3-bromo-propyne, 1,2-dichloropropane, and ethylene dichloride. Each compound suffers from drawbacks which greatly limit its commercial acceptability. For example, halogenated compounds are quite phototoxic, restricting their utility primarily to pre-planting treatments. In addition, halogenated compounds such as methyl bromide are quite expensive and have an adverse impact on stratospheric ozone. Methyl bromide applications on vineyards also may be ineffective in controlling vineyard soil pests due to the compound's inability to deeply penetrate heavy, coarse, or poorly prepared soils, as reported in USEPA, 430-R-96-021, 10 case studies, vol. 2, 1996.
Another class of compositions which have proven useful for controlling nematodes is thiocarbonates. U.S. Pat. No. 2,676,129 to Bashour describes the preparation of lower aliphatic disubstituted trithiocarbonates of the formula: ##STR1##
wherein R.sub.1 and R.sub.2 are alkyl radicals having from three to nine carbon atoms. Bashour describes dissolving the trithiocarbonate compounds in acetone for treating nematode-infested soils.
The use of sodium- and potassium thiocarbonates as effective nematode control agents is described in U.S. Pat. Nos. 2,836,532 and 2,836,533 to Seifter. The '532 patent relates to the use of sodium- and potassium trithiocarbonate. The '533 patent discloses alkali metal and ammonium salts of tetrathioperoxycarbonic acid.
One tetrathiocarbonate-based product, known commercially as ENZONE.RTM., is available from Entek Corporation, 1912 E. Lemon Heights Drive, Santa Ana, Calif. 92705, tel. (714) 731-5581. The active ingredient in ENZONE.RTM. is sodium tetrathiocarbonate. Sodium tetrathiocarbonate has received United States Environmental Protection Agency (USEPA) registration for grapes and citrus. Sodium tetrathiocarbonate's application to many other crops for controlling soil-borne diseases and pests also is being investigated. Sodium tetrathiocarbonate can be applied pre- or post-planting on vines and citrus. Sodium tetrathiocarbonate stoichiometrically degrades to carbon disulfide, sodium hydroxide, hydrogen sulfide, and sulfur in the soil.
The chemistry of thiocarbonic acids and their salts has been studied in some detail (O'Donoghue et al., J. Chem. Soc., 89(II), 1812, 1906; Yeoman, J. Chem. Soc., 119, 34, 192 1; Mills et al., J. Chem. Soc., 128(11), 2326, 1928; and Stone et al., U.S. Pat. No. 2,893,835). O'Donoghue et al. discloses preparing ammonium thiocarbonate by reacting liquid ammonia with cold alcoholic thiocarbonic acid. Thiocarbonic acid is said to be prepared by dropping a solution of calcium thiocarbonate into concentrated hydrochloric acid. The calcium thiocarbonate is described by the authors as a double salt, including the calcium cation in combination with both hydroxide and trithiocarbonate anions.
Yeoman describes the preparation of trithiocarbonates and tetrathiocarbonates (perthiocarbonates). Ammonium trithiocarbonate is said to be prepared by saturating an alcoholic ammonia solution with hydrogen sulfide, followed by adding carbon disulfide, and then adding dry ether to precipitate the salt product. Ammonium tetrathiocarbonate is said to be prepared in a similar manner, except that after reacting the ammonia and hydrogen sulfide, elemental sulfur is added to form the disulfide, (NH.sub.4).sub.2 S.sub.2, and carbon disulfide is added thereafter to form and immediately precipitate the desired tetrathiocarbonate product.
The works by O'Donoghue et al. and Mills et al., as well as the work by Yeoman, describe the instability of the salts of thiocarbonic acid. Yeoman observes that both ammonium trithiocarbonate and ammonium tetrathiocarbonate solutions are very unstable due to the decomposition of the salts into thiocyanate, as well as complete dissociation into ammonia, hydrogen sulfide, and carbon disulfide.
Yeoman further teaches that aqueous solutions of sodium trithiocarbonate and sodium tetrathiocarbonate remain stable only if oxygen and carbon dioxide are "rigidly excluded." The presence of oxygen is said to cause decomposition of the trithiocarbonate salts to carbon disulfide and thiosulfates, whereas carbon dioxide is said to decompose the trithiocarbonate salts to carbonate and carbon disulfide. Similarly, solutions of sodium tetrathiocarbonate are said to remain stable for a considerable time in the absence of oxygen and carbon dioxide. Oxygen is said to cause decomposition into thiosulfate and carbon disulfide. Carbon dioxide is said to decompose sodium tetrathiocarbonate to carbonate, elemental sulfur, carbon disulfide, and hydrogen sulfide. Potassium thiocarbonates are said to behave similarly.
Numerous efforts have been made to increase the stability of thiocarbonate salt solutions. Stone et al. describes the use of aprotic solvents such as hexane, cyclohexane, and benzene, or protic solvents such as ethanol, isopropanol, or dioxane to increase the stability of thiocarbonic acid salts.
Pilling et al., U.S. Pat. No. 5,039,327 discloses stabilized solid particles of one or more salts, thioesters, or complexes of trithiocarbonates in absolute ethanol. The solid particles are formed in a substantially water-free environment in which the thiocarbonate is said to be stable and substantially insoluble. The particles then are placed in an environment which is substantially free of water, CO.sub.2, and O.sub.2, and are encapsulated with a coating to protect them from future contact with air and water.
Pilling et al. describes the water-free environment in which the stabilized solids are prepared as one in which the water content is below the amount which would cause observable decomposition or hydrolysis of an unprotected thiocarbonate which is dissolved or suspended therein or which results in the formation and separation of an aqueous solution of the solids.
One process of preparing salts of tetrathiocarbonic acids is set forth in PCT/US96/11165. Salts of tetrathiocarbonic acids are said to be produced in a batch process in which a hydroxide, hydrogen sulfide, sulfur, and carbon disulfide are reacted sequentially to produce an aqueous solution of tetrathiocarbonate salts.
Several drawbacks exist with the batch process described in PCT/US96/11165. For example, the concentration of sodium tetrathiocarbonate produced typically is limited to about 32 wt %, mostly due to the low solubility of intermediate compound sodium sulfide. Conducting the reactions in the non-aqueous phase is said to unacceptably increase reaction time.
Another drawback is that extensive heat is generated during the exothermic reaction of hydroxide with hydrogen sulfide or its alkali metal salts. An efficient cooling system or a waiting period is necessary to avoid evaporation of the subsequently-added carbon disulfide. Alternately, a high pressure device must be used to keep the carbon disulfide in liquid form.
Yet another problem with the PCT '165 process is the formation of up to about 50 wt % trithiocarbonate salts. Trithiocarbonate salts are kinetically more stable than tetrathiocarbonate salts. The presence of trithiocarbonate reduces the effectiveness of tetrathiocarbonate as a time-controlled pesticide, nematocide, and fungicide.
Co-pending application Ser. No. 09/366,349 describes a process for preparing aqueous solutions of tetrathiocarbonate salts in high concentrations and in higher weight ratios with respect to trithiocarbonates. The '349 process comprises initially dissolving sulfur in carbon disulfide to form a sulfur solution. An alkali metal hydroxide solution is added to the sulfur solution, and an emulsion is formed. An alkali metal hydrosulfide is added to the emulsion under conditions sufficient to form the tetrathiocarbonate salt solution. While the process represents significant improvements over the prior art, it has been found that up to about 20 wt % of thiosulfates can be formed if the alkali metal hydrosulfide is added too rapidly or if temperature is not controlled. Thiosulfates undesirably reduce the quality of the end product and the yield of the desired tetrathiocarbonate salt.
It thus would be desirable to develop a manufacturing process for tetrathiocarbonate salts that provides aqueous solutions of tetrathiocarbonate salts in higher concentrations than now produced by prior art processes. It also would be desirable to develop a process that produces tetrathiocarbonates in higher weight ratios with respect to trithiocarbonates, and which avoids high pressures and minimizes the formation of hydrogen sulfide gas and undesirable solids, especially carbonates and thiosulfates. It also would be advantageous to produce tetrathiocarbonate salts at only slightly elevated temperatures and under atmospheric pressure to minimize the time and cost required for their preparation.